I started an excel spreadsheet to figure out the correct size battery bank and charging system. What became apparant at once is, the difficulty in estimating the power consumptipon of all the various power hungry devices we use. For example a Webasto 5500 air top heater says in the specs. that it will use 15/95 watts. ??? That is quite a range and obviously I must use a figure somewhere in between.

In theory if you are working on an average daily budget, so you estimate average daily usage.

For example...we have an 800W microwave. But if it gets more than 5 minutes of use in a day that is a lot and unusual. At 12V that uses up about 5.5 Ah total ((800/12) * (5/160)) - Peukert is studiously ignored!)

A computer might draw 5-6 Amps (call it 80W), but be on for 8-12 hours per day or even 24x7 if you are passage making. So that is going to take 40-72Ah off of your batteries even though the draw is a lot lower its the estimated time of consumption.

So you need to make some educated guesses about your average time on for all your devices to plug into the spreadsheet above.

I know it is complicated, but you could (or some say should) work your budget around a few scenarios. For us, our energy profile at anchor when the kids are using laptops, the microwave sees use, more cabin lights are on at night etc. Is VERY different than on a passage. We actually use a lot more power on passage as we have instruments, chartplotters, autopilots, etc. running on top of the refrigeration and other baseline systems.

So to get an accurate assessment you probably have two scenarios...three if you want to pick an "average" model.

1) At rest
2) Under way
3) "Average" model if you want only one

Plug in the numbers, make your estimates and make sure your batteries and chargers can handle both scenarios.

And underway for how long. You will be fine with 500Ah and 450W for at least a few days sailing in most cases and have more than enough at anchor. Also fine for longer crossings but you would need to be more energy conscious that usual and consider maybe having to run the engines for 2 hrs a day in neutral, using 2l of fuel, to keep the batteries topped up.

If you have a way to measure currentconsumption, that can help you fine-tune your numbers. My battery monitor/charger/inverter has an Ammeter I use to measure the consumption of my various devices. Make sure you turn off all charging sources when measuring current consumption.

Many devices will have an intermittent consumption (refrigerators, heaters, autopilots, electricwindlass, etc). You can watch the devices as they cycle to get an estimate of the average load. Remember that your refrigerator will cycle on more often in the tropics, and less often in cooler climates.

I've found that there are a few devices that use most of the power. At sea these will be chartplotter, computer, refrigerator, autopilot, nav lights, etc. My masthead light used to be the biggest drain (or close to it), until I switched to a LED tricolor. Likewise my computer. As technology has progressed I've been able to find good low-power computers, which has helped. I've been able to arrange my nav gear so I don't have to leave the chartplotter running 24/7.

My power budget has shown me where the low-hanging fruit is in my quest to run with minimal use of the engine for battery charging. Careful current measurements have made my power budget more accurate. Not perfect, but much better than a wild guess!

Helps me re-think things a bit, for example: that the chart plotter uses much more of the daily budget than I thought, what about durring long passages to switch it off for the most and only using it to verify once in a while (durring daylight hours)? Running the radar on standby for the most and transmitting on a regular schedule (every half hour for example)? On longer passages not running the fridge (dried foods and/or canned). On cloudy days doing more manual steering, etc.

There's also a good discussion from from knowledgeable people on that thread in regards to power consumption. As you can see the spreadsheet I created is the same one that sailorboy1 is now claiming to have created.

Just forget the spreadsheets and get a battery monitor, or better still a SmartGauge and an Ah meter that measures charging and discharging - then average the Ah discharge only that you get over 24 hours, both at anchor and under sail. Make sure you don't run the engine or have any charging from wind or solar during these 24 hr tests.

There's also a good discussion from from knowledgeable people on that thread in regards to power consumption. As you can see the spreadsheet I created is the same one that sailorboy1 is now claiming to have created.

I didn't claim to have created them. But to be clear I didn't "create" them just modified them for my use (must be important to someone)

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Helps me re-think things a bit, for example: that the chart plotter uses much more of the daily budget than I thought, what about durring long passages to switch it off for the most and only using it to verify once in a while (durring daylight hours)? Running the radar on standby for the most and transmitting on a regular schedule (every half hour for example)? On longer passages not running the fridge (dried foods and/or canned). On cloudy days doing more manual steering, etc.

Now I am getting inspired, thanks again!

Exactlyz Zai. One problem with our Raymarine plotter. It has a standby function, but won't go into standby when radar is running, even on timed transmit, which is unfortunate. That was my plan also crossing the Atlantic but we ended up running both a lot to track squalls and chewing a fair bit of power. Same with the fridge. We have two so the plan was use them both the first week then transfer the content,s if the second to the first and turn it off, then turn the second off after a week if need bel we ended up leaving both on all the way but it's good to plan different scenarios. Yes to manual steering and also making sure the ap doesn't work harder than it needs to by best sail trim. We have over 1 kW of solar and 800Ah batteries and at anchor we never go below 90% charge. Last boat had 600W and 540 Ah and also fine at anchor. I think around the same W solar as Ah batteries is a good rule of thumb. We had a wind gen before but not now.

It's an estimate. Trying to be too accurate is a waste of time because you will never have a day that exactly matches. Make some reaonable assumptions, add 20% (to cover new or missed items) and move on.